Electrostatic screen printing with magnetic conveyer and moving base electrode



Feb. 28, 1967 K. w. RAREY ETAL 3,306,193 ELECTROSTATIC SCREEN PRINTING WITH MAGNETIC CONVEYER AND MOVING BASE ELECTRODE Filed Sept. 14, l964 Y, INVENTQRS KENNETH ulRnREY @JOHN BKENMEDYAC ATTORNEYS United States Patent tiee 3,306,193 ELECTRSTATEC SCREEN PREN'HNG WITH MAGNETIC CUNVEYER AND MOVING BASE ELECTRDE Kenneth W. Rai-ey, South Holland, and John B. Kennedy,

Jr., Chicago, lill., assignors to Continental Can Company, Inc., N ew York, N .Y., a corporation of N ew York Filed Sept. i4, 1964, Ser. No. 396,060 11 Claims. (Cl. 10i-114) This invention relates to electrostatic screen printing are more particularly to a new and novel printing machine for use in high speed electrostatic screen printing processes.

In a previously known method of electrostatic screen printing, a stencil-supporting, electrically conductive screen is located near and parallel to a co-extensive sheet electrode, The screen and electrode may be either planar or curved into some simple shape, such as a portion of a cylinder or a cone. An object to be printed is located between the screen and sheet electrode. These must be sufficiently separated and appropriately shaped to acco-mmodate inclusion of the object therebetween. A high voltage, direct current power supply is connected between the screen and sheet electrode to establish and maintain an intense electric field therebetween. The screen and sheet electrode essentially constitute the plates of a parallel-plate capacitor.

The ink used in this previously known printing process is a finely-divided powder. This powder is made of a pigmented or dyed substance which generally is a good electrical insulator, has a low melting7 temperature, and is known as toner. The toner is applied to the side of the screen opposite to that facing the object to be printed. This application of toner to the scr-een may be done with a roller similar to the type used for painting. As the toner is pushed through unblocked apertures in the screen, the toner acquires a char-ge from the screen. The toner is then accelerated by the electric field between the screen and sheet electrode, and is moved toward the sheet electrode until intercepted by the object being printed. After a suiiicient amount of toner has 'been deposited, the flow is interrupted. The object may be then removed from its location between the screen and sheet electrode and may be exposed to heat oran appropriate solvent so as to produce permanent image adhesion to the object.

The invention to be disclosed hereinafter exhibits some similarity to the prior art just described. They are both stencilling processes, the stencils are preferably electrically conductive and are generally supported on conductive screens, bot-h processes can use the same toner particles, which toner particles are electrically charged for both pr-ocesses, intense electric fields are used to transport toner through space to result in printing, they are both suited to the same printing tasks, and the toner may be permanently affixed to the object or substrate being printed 'by identical methods.

Various differences also exist between the prior art and the present invention. The previously described method uses the conductive screen which supports the stencil for two functions: (l) charging the toner and (2) establishing and maintaining the electric vfield which carries the toner to the object to be printed. The invention to be hereinafter described, provides toner charging prior to passage of toner through the screen and without contact of toner with the screen; separating the functions permits control of each independently of the other.

The manner in which toner is introduced through the stencil apertures is of fundamental importance. Brushing toner through has several unique characteristics: (l) the toner comes through the screen at low velocity 3,366,193 Patented Feb. 28, 1967 which limits the object-screen separation; (2) at any instant, the toner comes through the screen over a limited area, thus limiting printing speed; (3) the uniformity of the toner distribution, in passing through the stencil, is determined by the distribution of toner in the brush and by the constancy of the brushing force and motion. By contrast, the present invention: (l) sends toner through the stencil at high velocity; (2) prints over an extended area (the entire pattern is printed simultaneously); and (3) exhibits very good uniformity of toner distribution over extended areas.

In practicing the prior art, it is absolutely necessary that an electric tield exists at the location in space where the toner meets the surface to be printed. Though desirable, such a eld is not absolutely necessary in this new approach.

Finally, when printing upon insulating materials, the accumulation of vtoner as the image is formed produces an electric field which opposes further deposition. It is desirable that charge tbe added to the object to be printed to minimize this field. The present invention considers and makes allowance for this phenomenon; the prior art methods exhibit a deficiency in this regard.

In general, the invention to be hereinafter disclosed is characterized by: (l) speed; (2) uniformity over extended areas; (3) good control of print density; (4) high resolution with large object-stencil separation; and (5) improved capability of printing upon insulating materials. in addition, the apparatus hereinafter described exhibits superior advantages in make-ready and clean-up time.

It is an object of this invention to provide a printing machine which is capable of performing a printing operation, a screen cleaning operation between printing operations, and a cleaning operation of the complete printing machine.

Another object of this invention is to provide a new and novel apparatus for electrostatic printing upon a substrate, the apparatus comprising a housing, a base electrode disposed adjacent to a portion of the housing, a stencil screen disposed between the substrate and the base electrode, supply means in the housing for supplying toner particles to the Abase electrode, and transfer means for transferring the toner particles from the base electrode to the substrate.

Another object of this invention is to provide new and novel apparatus, of the type described above, wherein the transfer means comprises means for establishing an electric field between the base electrode and the stencil screen to cause toner particles to be accelerated toward the stencil screen.

Another object of this invention is to provide new and novel apparatus, of the type described above, wherein the supply means comprises a depository in the housing, and magnetic means for transferring t-he toner particles from the depository to the base electrode.

Another object of this invention is to provide appalriatus, of the type set forth above, wherein the base electrode comprises an endless conductive belt, the supply means comprising a depository in the housing, and magnetic means for transferring the toner particles from the depository to the endless conductive belt.

Another object of this invention iis to provide apparatus, of the type described above, wherein the housing has a portion thereof defining an Kinclined plane, the inclined plane having apertures formed therein, ,a toner reservoir disposed in communication with the apertures, and means for transporting toner particles from the toner reservoir through the apertures.

Another object of this invention is to provide new and novel apparatus for electrostatic printing upon a substr-ate, the apparatus comprising `a base electrode, a

stencil screen disposed between the substrate and the base electrode, means for establishing an electric ield between the stencil screen and the base electrode, means for establishing ran electric eld between the substrate and the stencil screen, and control grid means for controlling the electric eld between the substrate and the stencil screen.

Another object of this invention is to provide, in ian electrostatic printing apparatus, a housing having a portion thereof being disposed to form an inclined plane, the inclined plane having apertures formed therein, a toner reservoir disposed in communication with the apertures, and means for transporting toner particles from the toner reservoir through the apertures.

Another object of this invention is to provide new and novel apparatus, of the type described above, wherein the Aapertures are provided by a mesh screen, and the means for transporting toner particles from the toner reservoir comprises a rotatable roller disposed in the toner reservoir adjacent to the mesh screen.

Another object of this invention is to provide, in an apparatus of the type utilizing a mixture of carrier particles and toner particles, a housing, a portion of the housing being disposed to form an inclined plane, the inclined plane having apertures formed therein, the apertures bein-g smaller than the carrier particles and larger than the toner particles, means for cascading carrier particles down the inclined plane past the apertures, and means for forcing toner particles through the apertures and into contact with the carrier particles.

Another object of the present invention is to provide apparatus for transporting and distributing magnetic particles, the apparatus comprising a housing, a portion of the housing defining a depository for the magnetic particles, a magnetic drum rotatably mounted in the housin-g adjacent to the depository, an endless belt mounted adjacent the magnetic drum and having a magnetic roller separated from the magnetic dimm by a portion of the belt, the magnetic roller having a Imore intense magnetic field than the magnetic drum whereby magnetic particles picked `up from the depository by the magnetic drum are transferred therefrom to the endless belt.

Another object of this invention is to provide, in an electrostatic apparatus utilizing carrier particles and toner particles, a base electrode, means for depositing carrier particles and toner particles onto the base electrode, a cleaning electrode, means for transporting the carrier par ticles 'and the toner particles from the base electrode to the cleaning electrode, and means for causing the toner particles to adhere to the cleaning electrode and the carrier particles to be expelled from the cleaning electrode.

A further object of this invention is to provide apparatus ot the type described above wherein the means for depositing the carrier particles land toner particles on the base electrode comprises a lmagnetic drum and a magnetic lroller, the magnetic roller having a more intense maignetic field than the magnetic drum, the base electrode comprising an endless belt, the belt having a portion thereof disposed between the magnetic drum and the magnetic roller, whereby the carrier particles and the toner particles are attracted from the magnetic drum toward the magnetic roller and deposited .upon the endless belt.

A still further object of this invention is to provide, in apparatus as described above, an inclined plane which extends downwardly from the endless belt toward the magnetic drum, whereby particles cascading down the inclined plane are directed toward the magnetic drum and attracted by its magnetic tield.

With the above and other objects in view that will hereinafter appear, the nature of the invention will be more clearly understood by reference to the following detailed description, the appended claimed subject matter and the several views illustrated in the accompanying drawing.

In the drawing:

FIGURE la is a diagriammatical illustration of a developer particle, of the type vused in electrostatic printing, and shows a plurality of small toner particles adhering to a larger carrier particle.

FIGURE lb is a diagrammatical illustration of fundamental apparatus ior performing an electrostatic screen printing operation, land shows a developer particle disposed upon a base electrode in spaced relation to a conductive stencil screen which is disposed in parallel relation to the base electrode and is connected in an clectrical circuit therewith such that an electric held may be produced therebetween.

FIGURE 1c is a diagrammatical illustration ol the printing apparatus upon closing of the switch to establish an electric field, and illustrates a developer particle being expelled from the base electrode and attracted toward the stencil screen.

FIGURE ld is a diagrammatical view of the printing apparatus and illustrates the carrier particle as being stopped by the stencil screen w .ile smaller toner particles are carried by their momenta through apertures in the stencil screen.

FIGURE le discloses a substrate being disposed in the path of the toner particles, and also illustrates the carrier particle as being expelled from the stencil screen and attracted toward the base electrode.

FIGURE lf is a diagrammatical illustration of the printing apparatus and shows the toner particles, which have passed through the stencil screen, as adhering to the substrate, and also shows the carrier particle on a return flight from the base electrode toward the stencil screen.

FIGURE 2 is a diagrammatical view of a printing machine embodied in the present invention.

FIGURE 3 is a diagrammatical view of a printing machine embodied in the present invention and illustrates apparatus for and one manner of cleaning the apparatus subsequent to a printing operation.

Referring to the drawing in detail, FIGURE la discloses a developer particle, generally indicated by the numeral 10, and includes a large carrier particle Il and a plurality of relatively small toner particles 12 which adhere to the carrier particle It by triboelectric forces. The carrier particle Il may be formed of electrically conductive material, such as charcoal, or metal shot of steel, aluminum, copper, etc.; however, in addition to being electrically conductive, it is preferred that the carrier particle 11 be formed of a ferromagnetic material such as iron filings. The toner particles I2 are preferably formed of a non-conductive material located in a triboelectric series such that the toner particles have a negative charge relative to the carrier particles ill, but the toner particles could also be charged positively with respect to the carrier particles.

In a practice according to the present invention, carrier particles Il were comprised of iron filings, to mesh (approximately 4 mils to 6 mils in diameter) supplied by Anken Film and Chemical Company for use in Electrofax magnetic brush developing systems. Toner particles used in the present invention are preferably, but not limited thereto, pigmented resin particles which are formed of n-butyl methacrylate (41%) and polystyrene (59%). Such toner particles are supplied by Xerox Corporation tor use in electrostatic olIice copiers and have an average particle diameter of I7 microns.

FIGURE 1b illustrates a single developer particle 10 as being disposed upon a base electrode 15 which is formed of conductive material. Located above thc base electrode 1S is a stencil screen i6 which is comprised of a wire mesh 17 having a conventional type ot coating 18 to define apertures 19 arranged in a predetermined pattern. Gne commercially available screen which is useful in the present invention is a 200 x 200 mesh stainless steel screen woven of 1.6 mil wire. Stencils conventionally used in silk-screen printing, have also been used.

The base electrode 15 and the stencil screen 16 are connected by suitable conductors to opposite sides-of a suitable high voltage source 22 and a suitable switch 23 is provided in one of the conductors. It is preferred, as shown, that the stencil screen 16 be connected to the positive side of the high voltage source 22, while the base electrode 15 is connected to the negative side of the high voltage source 22. The polarities may be reversed from those shown; however, the polarities shown are preferred for reasons which will become apparent from subsequent description. If the toner is positively charged with respect to the carrier, the stencil screen 16 is preferably connected to the negative side of the high voltage source 22.

As is shown in FIGURE lc, closing of the switch 23 establishes an intense electric eld between the base electrode 15 and the stencil screen 16, and the developer particle has acquired a net charge from the base electrode and is expelled from the base electrode 15 and is forced by the electric field toward the stencil screen 16. The net charging of the developer particle 10 occurs by direct electrical contact between the conductive carrier particle 11 and the base electrode 15. The conductive carrier particle 11 acquires the same charge as the conductive base electrode 15 and is, therefore, repelled therefrom. FIGURE ld illustrates a sequence of events which occurs at the time that the developer particle 10 is in contact with the conductive stencil screen 16. During the upward movement of the developer particle, as is sho-wn in FIGURE 1c, the developer particle 10 acquires a velocity such that upon collision of the carrier particle 11 with the stencil screen 16, toner particles 12 which are in alignment with an aperture 19 continue through the stencil screen 16 and rise thereabove. The carrier particle 11 is too large to pass through the small apertures in the stencil screen 16 and, because of contact with the conductive stencil screen 16, the carrier particle 11 acquires a charge at the screen which is opposite to the previous charge upon the carrier particle 11 and, therefore, is repelled by the stencil screen 16 and attracted toward the base electrode 15. An electric field may also be established between the stencil screen 16 and the substrate 25 by providingy a base electrode or control grid, such as the grid 41 which is shown in FIGURE 2, adjacent to the substrate on the side thereof which is remote from the stencil screen 16. Such a base electrode or control grid is held at the same polarity but at a higher potential than the stencil screen 16. As a result, such an electric field aids in propelling the toner particles toward the substrate 25.

FIGURES le and lf each illustrates a substrate 25 as being disposed above the stencil screen 16. After the irnpact of the carrier particle 11 with the stencil screen 16 has released some of the toner particles 12 from the carrier particle 11, momenta of the toner particles carry them into contact with the substrate 25 and the toner particles 12 adhere thereto. During the printing operation, the developer particles 1l) continue to oscillate back and forth between the base electrode 15 and the stencil screen 16 until sufiicient toner has been acquired upon the substrate 25 to produce a print of the desired optical density. The substrate 25 and the pattern thereon, which is formed by the toner particles 12 being deposited in a pattern which is determined by the apertures 19, are exposed to heat or vapors of a suitable solvent such that the toner particles are fused to the substrate 25.

Illustrated in FIGURE 2 is a basic printing machine, generally indicated by the numeral 28, which printing machine embodies the present invention. The printing machine 28 includes a housing 29 which is formed of a non-magnetic material, such as plastic, wood, or any other suitable material. An endless conductive ybelt 3f) is disposed adjacent to the top of the housing 29, and performs the functions as described above with reference to the base such as carbon tissue,

electrode 15. The housing 29 also includes a portion thereof defining an inclined plane 31 which provides a path for cascading developer particles 10 and/ or carrier particles 11 downwardly toward a portion of the housing 29 which defines a depository, generally indicated by the numeral 32.

A magnetic drum 33 is disposed adjacent to the depository 32 and may be mounted upon a shaft 34 and rotated by suitable drive means (not shown) in a counterclockwise direction as is indicated by the arrow in FIGURE 2. One operative embodiment of the present invention included a magnetic drum 33 which was comprised of an aluminum drum about 12 inches in diameter, which drum was covered by wrapping the entire curved surface with butting pieces of a flexible magnetic tape one inch wide and 1A@ inch thick, No. 60,208, purchased from the American Science Center of Chicago, Illinois. This was then wrapped with Mylar film, having a thickness of about 40 mils, so as to reduce the field intensity.

The conductive belt electrode 30 is suitably mounted upon a pair of rollers 36 and 37, either one or both of which are driven by suitable drive means (not shown). The roller 36 is a magnetic roller and is disposed adjacent to the magnetic drum 33 and separated therefrom by a portion of the conductive belt electrode 30. The magnetic roller 36 is designed so as to have a more intense magnetic eld associated therewith than the magnetic field which is associated with the magnetic drum 3'3. Consequently, developer particles 1f) or carrier particles 11, which` are picked up at the depository 32 by the magnetic drum 33, are transported by the magnetic drum to the conductive belt electrode 3f) and, because of the more intense magnetic field of the magnetic roller 36, the developer particles 1li or carrier particles 11 are transferred to the surface of the conductive belt electrode 30. As the electrode 3f) moves, it carries the particles away from the roller 36 and to a doctor blade 38 where the distribution and flow may be controlled. An overflow-turbe (not shown) returns excess particles from the doctoring zone back to the depository 32. An electrical connection is generally, though not necessarily, made to the conductive belt electrode 30 by lgoing through the doctor blade 38 and developer particles 10.

After doctoring, the developer particles 10 are transported into the printing zone where the oscillations of the particles between the electrode 30 and the stencil screen 16 occur, as were previously described in FIGURES lar-lf. Toner particles 12 which pass through the stencil apertures 19 are electrically transported to the substrate 25 being printed. The electric field which does this is located between the screen 16 and the substrate 25. At the beginning of the printing operation, it is preferable that a corona discharge occur behind the substrate 25. Corona wires 46 are provided for producing the desired corona discharge and an electric field is established be tween the stencil screen 16 and a control grid 41 to cause some of the ions formed by the corona discharge to be carried to the substrate 25. This establishes a high intensity electric field between the substrate 25 and the screen 16. In addition, the charge deposited on the substrate 25 tends to 'neutralize the field associated with a charged toner ima-ge which is caused by the accumulation of toner particles 12 upon the substrate 25. Thus, accumulation of toner particles 12 on the substrate 25 has little influence on subsequently arriving toner particles. In the case of a highly conducting substrate, an electrical connection made directly to it can accomplish both functions. For a reasonably conductive object, such as paper, a co-extensive electrode behind it can accomplish both. For these cases, no corona discharges and no control grids are required. But for insulating materials, such as cellulose acetate, the use of the corona discharge is preferred.

The arrangement described is preferred as a standard approach. For insulating materials, ions deposited on one side and toner particles, carrying charge of the opposite 7 polarity, deposited on the other side of the substrate, tends to form a double charge layer with only a small net external electric field. For conducting materials, conduction neutralizes the ions but also produces an excess or deficiency of electrons at the surface on which the toner particles are deposited, so that a similar result occurs.

After contributing to the printing operation, the carrier particles 11 cascade down the inclined plane 31 which returns them to the depository 32. On the way down the inclined plane 31, toner particles 1?. are added to replace those depleted by the printing operation. Mounted under the inclined plane is a toner reservoir 45 which is provided with a pivotally mounted cover 46. inside the reservoir 4S is a cylindrical brush 47, such as a paint roller, which brush is rotatably driven by suitable means (not shown). The brush 47 feeds toner particles from the reservoir 45 through apertures 48 in the inclined plane 31. A preferred construction is that the inclined plane 31 consist, at that portion adjacent the roller 47, of a piece of 200 mesh stainless steel wire cloth. The apertures 48 in this screen are large enough to pass toner particles 12 but are small enough to block passage of carrier particles 11 which are cascading down the inclined plane 31. While a brush 47 has been disclosed as the preferred means for forcing toner particles through the apertures 48, it is to be understood that other suitable means may be substituted for the brush 47, such as a gentle ow of air.

As the brush 47 is rotated, toner particles are forced through the screen apertures 48. As the carrier particles 11 roll across the screen, the toner particles are mixed with and adhere to the carrier particles. Subsequent cascading down the inclined plane 31 tends to improve mixing of the particles and contact between the carrier particles 11 and the toner particles 12 produces the triboelectric forces necessary for the toner particles 12 to adhere to the carrier particles 11. If desired, obstacles, such as deflecting plates, can be located on the inclined plane 31 to further enhance mixing. Thus, the carrier particles 11 are returned to the depository 32 with substantially the same amount of toner particles as when they left. Of course, the roller 47 is rotated at such a speed as to supply toner particles 12 in an amount substantially equal to the amount of toner particles being used for printing upon the substrate 25.

Carrier particles 11, of course, continuously circulate during painting with any particular particle only participating in perhaps every fourth or fifth print. While some are printing, some are being doctored, while other carrier particles are receiving additional toner particles.

The printing machine 28 may be provided with any suitable source of power. In one operative embodiment of the invention, a suitable power source was comprised of a Sorensen 230%2 P. R. & D. high voltage DC. power supply, with an output variable from to 30 kilovolts. During a printing operation, a potentiad difference of approximately 12 kilovolts was maintained between the stencil screen 16 and the conductive belt electrode 30, with the screen being positive relative to the electrode 3f). It is preferable that the stencil screen 16 be cleaned between each printing operation and, for such cleaning operation, a voltage of 12 kilovolts, or slightly less, was maintained with the screen being negative relative to the base electrode 30.

As is illustrated in FIGURE 2, high voltage sources 50, 51, 52 and 53 are provided. The high voltage source 52 maintains a potential difference of approximately 12 kilovolts between the base electrode 30 and the stencil screen 16, as set forth above. The voltage source 50 may be varied Ibetween 50() to 1000 volts for maintaining the control grid 41 at such potential difference above the stencil screen 16 while the corona electrodes 40, which are comprised of two and three mil platinum wires, have been operated about kilovolts above the control grid 41 by high voltage source 51. The stencil Cit screen 16 has been spaced approximately 1/2 inch above the conductive belt electrode 30, the control grid 41 has been spaced approximately 1/2 inch above the stencil screen 16, and the corona electrodes 4l) have been spaced approximately 1/2 inch above the control grid 41.

The control grid 41 is preferably comprised of a 16 x 16 mesh bronze screen woven of wire about 23 mils in diameter. The purpose of this control grid 41 is to control the flow of ions formed by the corona discharge, at the corona electrodes 40, such that some of the ions formed in this discharge are carried to the substrate 25.

if desired, suitable switches 55, 56 and 57 may be provided for simultaneously opening and closin gthe circuitry containing the high voltage sources Si), 51 and 52. Such switch-es 55, 55 and 57 may be connected together by a suitable mechanical connection which is illustrated by the dotted line 53.

In order to effect a cleaning operation of the stencil screen 16, between each printing operation, a high voltage source 53 may be supplied. The high voltage source 53 can establish a potential difference between the conductive base electrode 30 and the stencil screen 16 of l2 kilovolts, or slightly less, and is, therefore, comparable to the high voltage source 52 except that a normally opened pushbutton 59 places the high voltage source 53 into the circuit such that the polarities of the conductive belt electrode 30 and stencil screen 16 are reversed.

A further explanation of the cleaning operation is required. During the cleaning operation, the stencil screen 16 is maintained at the same polarity as the triboelectrically charged toner particles. In the cited example, this would be negative. The field intensity between the screen 16, and the belt 3f), is suiciently high that the developer particles oscillate vigorously between the screen and belt similarly to their motion during printing. However, the electric field is oppositely directed to that used for printing. The carrier particles mechanically dislodge accumulated toner from the screen when they strike it during the oscillations. Once dislodged, the electric field carries the toner away from the screen. Some toner can be removed with only .a high intensity electric Afield of appropriate direction. However, this has not been found adequate for repetitive printing. supplementing the field with the oscillating carrier particles, though, has been found to be completely adequate.

Some consideration must be given to both the intensity of the field during cleaning and the time permitted for the operation. If either of these are excessive, disadvantages may be encountered. The motion of the carrier particles during cleaning is quite comparable to that during printing. Thus, the carrier is delivering toner to the screen as it does during printing. Toner surrendered at the screen that has suf'licient momentum and is appropriately directed will proceed on through the stencil apertures and produce printing. However, a field intensity suficient to produce cleaning and yet insufficient to produce appreciable printing may be found. In general, it is desirable to clean with a lower field intensity than that used for printing so as to obtain this condition. Now, in addition, as long as the field is sufficiently intense to produce carrier oscillations, some printing necessarily occurs. Even with minimal carrier motion a high density print can be obtained if continued for a sufficient time, such as several minutes. However, the times actually used for cleaning typically are from 0.1 second to 0.4 second. By using a sufficiently small field intensity and a sufiiciently short cleaning time, a screen can be exceptionally well cleaned and yet no readily discernible amount of toner be deposited on a substrate located in the printing position. Thus, objects may be moved into and out of the printing position while screen cleaning is occurring.

If the printed substrate is to be maintained in the printing location during the cleaning operation, then considerations of time and field intensity are less significant. However, by proper attention to the values permitted these variables, one can obtain the benefits of a cleaning operation that is substantially free of simultaneous printing.

Illustrated in FIGURE 3 is a feature of the present invention wherein the carrier particles 11 can be used to clean the printing machine 28 at the end of a printing task. A continuous electrode 60, having a continuous conductive surface 61, may be substituted for the stencil screen 16 and can be used to remove toner particles from the carrier particles 11. This is done by maintaining the cleaning electrode 60, by the high voltage source 62, at substantially the same potential as the stencil screen 16 when printing is occurring. The electric field between the cleaning electrode 60 and the conductive belt electrode 30 causes the carrier particles to oscillate back-andforth between them. Astoner particles are freed by carrier collisions, they are attracted to the cleaning electrode 60. In this manner, toner particles may be removed from the carrier particles in substantial quantities. The collected toner particles can be continuously removed from the cleaning electrode 60, by a suitable scraper 63, and deposited in a toner recovery bin 64.

Cleaned carrier particles 11 are recirculated through the printing machine 28 and will pick up any toner particles that may have collected on surfaces in the printing machine itself. By maintaining this cleaning and recirculating operation for approximately several minutes, substantially all cleaning required to change toner colors can be automatically and expeditiously accomplished. Of course, during the cleaning operation, the brush 47 is maintained at rest so as not to add additional toner particles to the system.

While preferred forms and arrangement of parts have been shown in illustrating the invention, it is to be clearly understood that various changes in details and arrangement of parts may be made without departing from the spirit and scope of the invention, as defined in the appended claimed subject matter.

We claim:

1. Apparatus for electrostatic printing upon a substrate, said apparatus comprising a housing, a base electrode disposed adjacent to a portion of said housing, a stencil screen disposed between said substrate and said ba-se electrode, supply means in said housing for supplying conductive carrier particles and toner particles to said base electrode, means for moving said base electrode with said carrier particles and toner particles relative to said stencil screen and said housing, and transfer means for transferring said toner particles from said base electrode to said substrate; said supply means comprising a depository in said housing, and magnetic means for transferring said conductive carrier particles and toner particles from said depository to said base electrode.

2. Apparatus for electrostatic printing upon a substrate, said apparatus comprising a housing, a base electrode disposed adjacent to a portion of said housing, a stencil screen disposed between said substrate and said base electrode, supply means in said housing for supplying toner particles to said base electrode, and transfer means for transferring said toner particles from said base electrode to said substrate; said base electrode comprising an endless conductive belt; said supply means comprising a depository in said housing, magnetic means for transferring said toner particles from said depository to said endless conductive belt.

3. Apparatus for electrostatic printing upon a substrate, said apparatus comprising a housing, a base electrode disposed adjacent to a portion of said housing, a stencil screen disposed between said substrate and said base electrode, supply means in said housing for supplying toner particles to said base electrode, and transfer means for transferring said toner particles from said base electrode to said substrate; said housing having a portion thereof defining an inclined plane, said inclined plane having apertures formed therein, a toner reservoir disposed in communication with said apertures at an undersurface of said inclined plane, and means for transporting toner particles from said toner reservoir through said apertures.

4. Apparatus for electrostatic printing upon a substrate, said apparatus comprising a base electrode, a conductive stencil screen disposed in spaced relation between said substrate and said base electrode, means for establishing a first electric field between said stencil screen and said base electrode, means for establishing a second electric field between said substrate and said stencil screen, and control grid means for controlling said second electric field between said substrate and said stencil screen. v

5. Apparatus for electrostatic printing upon a substrate, said apparatus comprising a base electrode, a stencil screen disposed between said substrate and said base electrode, means for establishing an electric eld between said stencil screen and said base electrode, said means comprising a high voltage energy source, conductor means connecting one side of said source to said stencil screen, a conductive doctor blade means for distributing toner particles on said base electrode and being disposed in electrical contact with said base electrode, and conductor means connecting the other side of said source to said conductive doctor blade.

6. In an electrostatic printing apparatus, a housing, a portion of said housing being disposed to form an inclined plane having an upper end and a lower end, said inclined plane having apertures formed therein along a portion thereof intermediate said upper and lower ends, a toner reservoir disposed in communication with said apertures at an undersurface of said inclined plane and means for transporting toner particles from said toner reservoir through said apertures.

7. In an electrostatic printing apparatus, as defined in claim 6, wherein said apertures are provided by a mesh screen, and said means for transporting toner particles from said toner reservoir comprises a rotatable roller disposed in said toner reservoir subadjacent to said mesh screen.

8. In an apparatus of the type utilizing a mixture of carrier particles and toner particles, a housing, a portion of said housing being disposed to form an inclined plane, said inclined plane having apertures formed therein, said apertures being smaller than said carrier particles and larger than said toner particles, means for cascading carrier particles down said inclined plane past said apertures, and means for forcing toner particles through said apertures and into contact with said carrier particles.

9. Apparatus for transporting and distributing magnetic particles, said apparatus comprising a housing, a portion of said housing defining a depository for said magnetic particles, a magnetic drum rotatably mounted in said housing adjacent to said depository, and endless belt mounted adjacent said magnetic drum and having a magnetic roller separated from said magnetic drum by a portion of said belt, said magnetic roller having a more intense magnetic field than said magnetic drum whereby magnetic particles picked up from said depository by said magnetic drum are transferred therefrom to said endless belt.

10. Apparatus as defined in claim 9 wherein said housing has a portion defining an inclined plane, said inclined plane extending from adjacent said belt to said depository, whereby magnetic particles delivered to said inclined plane from said belt are cascaded down said inclined plane and returned to said depository.

11. Apparatus as defined in claim 10 wherein said inclined plane is provided with a plurality of apertures which are smaller in size than said magnetic particles, a toner reservoir for toner particles of a size smaller than said apertures, said toner reservoir being disposed in communication with said apertures and having means for forcing toner particles through said apertures and into contact with said magnetic particles as said magnetic particles cascade down said inclined plane.

References Cited by the Examiner UNITED STATES PATENTS Schaert 101-426 Mayo et al. 11S-637 Sugarman 118-637 X Haas.

Wilson 118-637 X Mayo.

Fisher et al. 118-637 Watson.

Kulesza.

Remer l5-1.5 Codichini et al. 118-637 X ROBERT E. PULFREY, Prima/'y Examiner.

1D E. S. BURR, Assistant Examiner. 

1. APPARATUS FOR ELECTROSTATIC PRINTING UPON A SUBSTRATE, SAID APPARATUS COMPRISING A HOUSING, A BASE ELECTRODE DISPOSED ADJACENT TO A PORTION OF SAID HOUSING, A STENCIL SCREEN DISPOSED BETWEEN SAID SUBSTRATE AND SAID BASE ELECTRODE, SUPPLY MEANS IN SAID HOUSING FOR SUPPLYING CONDUCTIVE CARRIER PARTICLES AND TONER PARTICLES TO SAID BASE ELECTRODE, MEANS FOR MOVING SAID BASE ELECTRODE WITH SAID CARRIER PARTICLES AND TONER PARTICLES RELATIVE TO SAID STENCIL SCREEN AND SAID HOUSING, AND TRANSFER MEANS FOR TRANSFERRING SAID TONER PARTICLES FROM SAID BASE ELECTRODE TO 